In boat fishing or the like, the success of a catch is directly affected by the accuracy with which a fishhook or the like is placed in a school of fish detected by a fish finder. To this end, there are several types of conventional fishing reels and water depth measuring device for measuring the wound or unwound length of a fishline. Each of the conventional water depth measuring device includes an electronic control section (main section) combined with a rotation transmitting section (mechanical section), and it is not easy to assemble and disassemble the water depth measuring device when repairing or replacing an element thereof. Also, it has been difficult to manufacture a water depth measuring device that is completely waterproof and not subject to water damage.